1. Field of the Invention
The present invention relates to a color reading apparatus for and a color reading method of reading color signals on the basis of a R light beam, a G light beam and a B light beam by use of light sources and a charge coupled device. The present invention is effective especially in a color scanner and a color copier which employ gradation data.
2. Description of the Related Art
The color scanner is generally used as a color reading apparatus. This color scanner has a photoelectric converting device such as a charge coupled device (hereinafter abbreviated to a CCD). This CCD accumulates an electric charge through a beam of light reflected by a copy and reads the accumulation electric charge in the form of a color signal. When this CCD is employed for the color reading apparatus, a quantity of the accumulated electric charges is small for a quantity of light received from the copy. That is, a sensitivity of the CCD is poor. For this reason, a variety of color reading apparatuses are adopted to enhance the sensitivity of this CCD.
These color reading apparatuses are, e.g., a color filter switching apparatus, a color CCD apparatus and a prism separation apparatus.
First, the color filter switching apparatus includes a hot-cathode ray tube or a cold-cathode tube for irradiating a copy with a beam of white light and a mirror for reflecting the light reflected from the copy toward a lens. The color filter switching apparatus has a CCD for receiving the light from the lens and color filters, disposed between the mirror and the lens, for separating the light. Then, the color filter switching apparatus separates the white light of the hot-cathode ray tube which exhibits a large light quantity into the R light beam, the G light beam and the B light beam by switching over the above color filters in the sequence of R, G and B. This color filter switching apparatus is classified into two types, i.e., a slide type and a rotary type.
A scanning method through the CCD is classified into two methods, viz., a linear sequential scanning method and a surface sequential scanning method. The linear sequential scanning method is a method of switching over the filter every time a one-line scan is effected on the copy. The surface sequential scanning method is a method of switching over the filter per surface of the copy and simultaneously performing the scan three times. According to this scanning method, a CCD for monochrome is employed as it is, and, therefore, the costs become comparatively low. This scanning method, however, has a drawback in which a reading speed is relatively slow.
Next, in the color CCD apparatus, the color filters transmitting a RGB light beam are provided on the surface of the CCD. These color filters work to separate the white light reflected from the copy into the R light beam, the G light beam and the B light beam, respectively.
This color CCD apparatus is capable of reading the accumulation electric charge at a high speed. Further, down-sizing of the apparatus is also attainable. There exist, however, such defects that a focal depth thereof is small, and scatters in terms of sensitivity are produced per sensor and per color.
Besides, the prism separation apparatus incorporates a lens, CCDs for respective light beams and a prism disposed between the lens and the CCDs. Then, a refractive index of the prism varies corresponding to respective wavelengths of the R light beam, the G light beam and the B light beam, and hence the reflected/refracted light from the copy can be separated into the respective light beams. This prism separation apparatus is capable of reading the accumulation electric charge at the high velocity and has a large focal depth.
An optical system thereof has to be, however, designed with a high accuracy so that the CCDs for the respective light beams read the same copy surface at all times. Further, the prism separation apparatus presents defects in which the costs become relatively high, and the apparatus increases in size.
Moreover, the above-mentioned color filter switching apparatus, the color CCD apparatus and the prism separation apparatus have the following problems. In these apparatuses, if the hot-cathode ray tube and the cold-cathode tube having the white light are employed as light sources, a stabilizing time of a rising light quantity increases. The temperature changes linearly with respect to the light quantity, and the light source is blackened due to the use of mercury. There is also a defect in which the infrared ray augments because of argon gas when the temperature is low.
Further, the light source of the hot-cathode ray tube or the cold-cathode tube is bad in terms of the rising characteristic of the light quantity. The light source has to be always in an operating status while reading a copy for one page.
Under such circumstances, a rare gas fluorescent lamp obviating the defects given above has been employed in recent years. The rare gas fluorescent lamp has a smaller light quantity than the light quantity of the hot- or cold-cathode tube. The rare gas fluorescent lamp is better in terms of the rising characteristic of the light quantity than in the hot- and cold-cathode tubes. For this reason, the rare gas fluorescent lamp has been used for a RGB three-light-source switching apparatus.
The three-light-source switching apparatus separates the R light beam, the G light beam and the B light beam by sequentially switching over the R light source, the G light source and the B light source. This apparatus obtains a color signal composed of a R signal, a G signal and a B signal from the R light beam, the G light beam and the B light beam through the same CCD. Accordingly, there disappear problems in terms of an image matching and a color difference.
The color reading apparatus using the three-light-source switching apparatus is, however, incapable of taking gradations of a low-density region and a high-density region due to an afterglow of the fluorescent lamp and an afterglow of the CCD. Hence, the image obtained exhibits an ill-contrasted image quality on the whole.
Supposing, for example, that the copy changes from a bright portion (while) to a dark portion (black), it is desirable that an analog output of the CCD in this case be, e.g., 100 in the bright portion but be 0 in the dark portion.
However, the afterglow of the CCD is on the order of several % (2 through 8%) of the light accumulated in the CCD. For instance, the afterglow of the CCD is 5% of the light accumulated therein, the CCD output changes from 100 to 5. For this reason, a color is produced in the dark portion of the CCD output when the copy changes from the bright portion to the dark portion. Alternatively, in reverse to this, the color is produced in the bright portion. Consequently, the contrast decreases on the whole.
Further, the decrease in the contrast is caused even by the afterglow of the light source. That is, the problem is that the accurate CCD output can not be obtained because of the afterglow output being superposed on the original CCD output due to the afterglows of the light source and of the CCD.